Cementitious exterior sheathing product with rigid support member

ABSTRACT

Cementitious exterior sheathing products are provided which include a rigid support member affixed to a cementitious layer. The rigid support member includes at least one nailing flange disposed along one of its lateral sides for allowing the sheathing product to be affixed to an exterior wall of a building. Preferred mechanical and adhesive bonding techniques are suggested for combining the cementitious layer and rigid support member together to form an integrated product. Such products are lighter in weight and are more crack resistant than currently available fiber cement trim boards.

FIELD OF THE INVENTION

[0001] This invention relates to exterior sheathing products whichincorporate cementitious materials, and especially, fiber cementsheathing which is lighter and more resistant to cracking when fastenedto the exterior walls of buildings.

BACKGROUND OF THE INVENTION

[0002] Fiber cement has been used in the United States buildingmaterials industry since the 1980's. This material is used inresidential and commercial construction applications as an alternativeto wood for siding, roofing, backer board, trim and fascia applications.Fiber cement is fire and insect resistant, and is more durable. In fact,it was the fastest growing market segment in the exterior sheathingindustry in the 1990's, and by 2005, this material is expected to gainup to 25 percent of the siding market.

[0003] Fiber cement is, technically, a composite of portland cement,aggregate (usually sand), and cellulose fibers. Cellulose fibers areadded to cement to increase its toughness and crack-arresting ability.Fiber cement shingle and shake products are widely available from suchsources as James Hardie, Inc. under the brand name Hardiplank® andCertainTeed Corporation under the brand name Weatherboards™. Theseproducts are produced by the Hatchek de-watering process, which resultsin a laminated flat sheet reinforced with a significant amount ofcellulose fibers, usually about 30-35 percent by volume.

[0004] Fiber cement materials possess useful properties, but they wereat one point in their history believed to be unsuitable for exterior usesince they were susceptible to damage due to the effect of freeze-thawcycles. See Harper et al., U.S. Pat. No. 4,637,860. Freeze-thaw actioncan cause severe deterioration to fiber cement building products. Theprimary cause of damage is due to the hydraulic pressures that developas water freezes and expands in tiny fissures and pores of cementitiousmaterials. Once these forces exceed the strength of the material,cracking occurs. During subsequent thawing, the water then moves throughthe cracks, expanding them further, to cause more damage when freezingoccurs again.

[0005] Harper et al., U.S. Pat. No. 4,637,860, suggested that betterfreeze-thaw resistance could be achieved by autoclaving a cellulosefiber cement mixture with silica sand additions. These inventors alsorecognized that silica sand additions reduced the density of formedsheet materials to a level below that necessary to achieve sufficientstrength and freeze-thaw resistance. Accordingly, the '860 patentsuggested compressing the wet mixture in a press to reduce its thicknessand increase its density prior to autoclaving. Such a process has beenproven to be effective in increasing the interlaminar bond strength(“ILB”) of fiber cement boards when pressures approaching 30 bar areused. See Wierman et al., “The Effects of Pressure on Freeze-ThawDurability of Fiber-Reinforced Cement Board” (______ , 2002).

[0006] While improvements to the processing of fiber cement sheathinghave been introduced, there have been some notable disadvantagesassociated with fiber cement products compared to vinyl siding products.Specifically, even with cellulose fiber reinforcement, fiber cementpanels and trim boards are susceptible to cracking by nails and screws,especially along their edges. Moreover, fiber cement trim boards are farheaver than their vinyl counterparts, and can be difficult to handle.

[0007] Accordingly, there is a current need in the cementitious exteriorsheathing industry for a lighter weight, more crack resistance, andtherefore, more durable siding and trim panel.

SUMMARY OF THE INVENTION

[0008] The present invention provides, in a first embodiment, acementitious exterior sheathing product, including a rigid supportmember having a wall-facing side and an exterior-facing side. The rigidsupport member also includes a pair of lateral sides, a pair oflongitudinal ends, and at least one nail flange disposed along one ofits lateral sides. Disposed on a portion of the exterior-facing side ofthe rigid support member is a cementitious layer which exhibits anaesthetic appearance.

[0009] The cementitious exterior sheathing products and fiber cementtrim boards of this invention can be provided in reduced thicknesses, ofabout 0.31 in., for example, which would be significantly lighter, andeasier to carry than the 1 inch thickness trim boards currently providedin the marketplace. By transferring the load to a rigid support member,the cementitious trim boards of this invention can be made as much as50% lighter than currently available fiber cement trim boards. The rigidsupport members of this invention are preferably provided with fastenerreceiving holes so that they, and not the fiber cement material, absorbthe stress of hammering.

[0010] The preferred rigid support members of this invention includeresinous or metallic materials which act as a reinforcement to thecementitious layers. These materials, while rigid and possessing agreater flexural modulus than fiber cement, can, nevertheless, bepre-perforated, perforated by the fasteners used to hang the trimboards, or provided as a lath structure, to enable nail or screwfastening with minimal stress, as well as, provide mechanical locking orbonding between the cementitious material and the rigid support.Additionally, cement bond promoters, such as polyvinyl-acetate (“PVA”)and acrylic coatings, may be used to provide an adhesive bond betweenthe cementitious layers and the rigid support members of this invention.

[0011] In a further embodiment of this invention, a fiber cement trimboard is provided which includes an elongated rigid support memberhaving a wall-facing side and an exterior-facing side, a pair of lateralside portions and a pair of longitudinal ends. The exterior-facing sideof the rigid support member includes at least a first and a secondexterior wall portion. Disposed along a first of the pair of lateralside portions of the rigid support member is a first nailing flange, anda fiber cement layer is disposed on a portion of the exterior-facingside of the rigid support member. The fiber cement layer exhibits anaesthetic appearance. In the preferred embodiment, the fiber cementlayer is a corner trim board.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure, inwhich:

[0013]FIG. 1: is a front perspective view of a corner trim board of thisinvention;

[0014]FIG. 2: is a diagrammatic, partial view of a manufacturingtechnique, including the de-watering of a plurality of cementitiouslayers;

[0015]FIG. 3: is a flow diagram of a preferred Hatchek and pressingprocess for manufacturing the trim board and sheathing products of thisinvention;

[0016]FIG. 4: is a diagrammatic front perspective view of the stackingof fiberglass molds, wet cement boards and rigid support members, priorto pressing;

[0017]FIG. 5: is a partial cross-sectional side view of a portion of thestacked structure of FIG. 4, showing the alternate layers of fibercement, fiberglass mold and rigid support members;

[0018]FIG. 6: is a diagrammatic side view of a pressing operation; and

[0019]FIG. 7: is a diagrammatic side perspective view of a pressingoperation for a corner trim board.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Cementitious exterior sheathing products, such as lap siding,shingles, panels, planks, vertical siding, soffit panels, fascia, cornerpost, column corners and trim boards, are provided by this invention. Asused herein, “cementitious” refers to hydraulically settable materials,including a hydraulically settable binder, such as hydraulic cement,gypsum hemihydrate, and calcium oxide, and water to which componentssuch as aggregates, fibers, dispersants and a rheology-modifying agentscan be selectively added to modify the properties of the mixture. Theterm “fiber cement” refers to a cementitious composition includingportland cement, cellulose fibers and sand.

[0021] With reference to the figures, and particularly FIGS. 1-3thereof, a process for making cementitious exterior sheathing products,such as a corner trim board 100, is disclosed. In a first embodiment ofthis invention, shown in FIG. 1, a corner trim board 100, is provided.The corner trim board 100 includes an elongated rigid support member 11having a wall-facing side 20 and an exterior-facing side 10, a pair oflateral side portions and a pair of longitudinal ends. Theexterior-facing side 10 includes at least a first and a second exteriorwall portion 14 and 16, respectively. The corner trim board 100 furtherincludes a first nailing flange 18 disposed along at least a first ofthe pair of lateral side portions, and a cementitious layer 22,preferably made from fiber cement, disposed at least on a portion of theexterior-facing side 10 of the rigid support member 11. The cementitiouslayer 22 exhibits an aesthetic appearance, such as a wood grain. Thecementitious layer 22 can also take the shape of decorative panels,including lap and full cut designs with deep, authentic textures, suchas lap shingles, random square straight edges, random square staggerededges, half rounds, and octagons.

[0022] The corner trim board 100 can further include a second nailingflange 19, the first and second nailing flanges 18 and 19 havingdisposed therethrough a plurality of fastener receiving holes 32. Thefastener receiving holes 32 are desirably elongated slots through therigid support member 11, although with some sacrifices, fastener holescan also, or alternatively, be located through the cementitious layer22. The corner trim board 100 further includes, in this embodiment,third and fourth exterior wall portions 26 and 28 connected to the firstand second exterior wall portions 14 and 16 respectively. The first,second, third and fourth exterior wall portions 14, 16, 26 and 28 aredesirably arranged such that the first and fourth wall portions 14 and28 are substantially parallel and the second and third wall portions 16and 26 are substantially parallel. More preferably, the first nailingflange 18 is also substantially parallel to the first wall portion 14and a second nailing flange 19 is substantially parallel to the secondexterior wall portion 16, in this embodiment.

[0023] Alternatively, the rigid support member can take the form of arectangular enclosure having first and second pairs of generallyparallel walls (14, 28 and 16, 26 respectively), and a longitudinalslotted opening 23.

[0024] The cementitious layer 22 is disposed at least over theexterior-facing side 10 of the rigid support member 11, preferably priorto pressing and autoclaving. A good mechanical bond can be providedbetween the rigid support member 11 and the cementitious layer 22 byperforating the rigid support layer with through-holes 33 which allowthe wet cementitious material to bulge through the rigid support member11, as shown in FIG. 1., and following setting, provide it with amechanically locking engagement. A plurality of like through-holes canbe uniformly distributed through the first and second exterior wallportions 14 and 16, for example. Alternatively, or additionally, aresinous cementitious bond promoter 30 can be applied to any surface ofthe rigid support member 11 intended to be in contact with thecementitious layer 22, such as the first and second exterior wallportions 14 and 16, to promote an adhesive bond with the cementitiouslayer 22. Cementitious bond promoters include PVA and acrylicformulations, such as the acrylic-based cement mortar modifier Rhoplex®E-330, available from Rohm & Haas Company, Philadelphia, Pa.Additionally, the through-holes 33 can be punched through metal sheet orplate so as to provide one or more extending prongs 21 which can furtheraid in the mechanical locking of the cementitious layer 22 to the rigidsupport member 11 by extending into the cementitious layer 22 prior tocuring.

[0025] The preferred corner trim board 100 of this invention enables theuse of thinner cementitious materials than those formerly suggested forcorner trim applications. The corner trim board 100 of this inventioncan include, for example, an cementitious layer 22 which isapproximately at least about 0.05 inches, preferably about 0.25-0.75inches, and more preferably about 0.31-0.63 inches in thickness, asopposed to as much as one inch thick prior art constructions.Additionally, since the fastener receiving holes 32 are preferablylocated in the rigid support member 11, nailing can be accomplishedwithout risking the development of cracks or over-stressed areas in thecementitious layer 22. The corner trim board 100 of this invention canbe designed to save at least 10 percent of the weight, and as much as 50percent of the weight, of conventional trim board constructions withoutsacrificing strength or durability. In fact, due to the mechanicaland/or adhesive bonding between the rigid support member 11 andcementitious layer 22, and the potential for “re-bar”—like reinforcementof the cementitious layer 22, the present construction may be evenstronger.

[0026] With regard to the process for manufacturing trim boards andsheathing products of this invention, reference is now made to FIGS.2-7. The steps for manufacturing fiber cement are well known. First, thedry ingredients are mixed thoroughly in a mixing step 210 of FIG. 3.These ingredients typically include, aggregates, fibers, dispersants,and a hydraulically settable binder, such as cement. A typicalconstruction will include portland cement, sand, cellulose fibers,kaolin and performance additives in less than one percent by weight.These dry ingredients are next mixed with water and formed into a slurryat step 212. The slurry 203, shown in FIG. 2, is deposited bydisplacement screws to a series of tanks 201 which are fluidly connectedto a series of transfer (or pick up) rolls 202. Agitators 209 disposedin the tanks 201 help to mix the fiber cement slurry 203 to keep theingredients from settling. The transfer rolls 202 and pressure rolls 211apply a thin layer of slurry 203 to the felt belt 213. As this belt 213passes by each tank 201 it “picks up” a little more slurry until thefull wet thickness of each layer 204 is achieved.

[0027] The Hatchek process 214 includes a dewatering station or vacuumbox 207 for de-watering each layer 204 of cementitious material and anaccumulator roll 205 for assembling the layers together into about a0.25-0.75 inch, preferably about 0.31-0.63 inch (dry cured thickness),multi-layered sheet 206. The cementitious sheet 206 is roll formed atstep 216 through one or more further nip rolls or belts, followed by acutting step 218. In a preferred embodiment, the multi-layered sheet 206is wet or shear cut at step 216 to provide a 5 ft. wide×12 ft. longsheet dimension. The cutting step 216 can be accomplished by a water jetor cutting knife 306 as shown in FIG. 4.

[0028] At the layering step 220, which is diagrammatically illustratedin FIGS. 4 and 5, the rigid support member 302 and suitable fiberglassmold 303, containing, for example, an aesthetic wood grain or the like,are joined with the cut board 301 in a stack 304.

[0029] The pressing and final de-watering of the product is accomplishedat step 222, shown also in FIG. 6, in which great pressures of about 500psi (30 bar) are applied by a 16,000 ton press for approximately onehour. The pressing and de-watering step 222 should be sufficient tosqueeze water out of the cementitious material and impress an ornamentalappearance from the fiberglass mold 303 into the cut board 301.

[0030] The disclosed Hatchek process 214, can be modified to introducean interlaminar bond strength (“ILB”) promoter through spray head 208.Such an ILB strength promoter could include, for example, a cementmortar modifier or adhesive, such as acrylic, or PVA, such as theaforementioned Rhoplex® E-330, for assisting in promoting betteradhesion between the layers 204 of the multilayered sheet 206.Alternatively, or additionally, a rheological agent, such as“nano-sized” magnesium alumino silicate, can be sprayed onto the fibercement layer prior to sheet formation at the accumulation roll 205, toencourage flow between the individual cementitious layers 204 to createbetter interlaminar bond strength. One rheology-modifying agent used toproduce such an effect is the Acti-gel™ brand of activated magnesiumalumino silicate available from Active Minerals Company LLC, HuntValley, Md. Such rheological agents and cement adhesive promoters canalso be added to the slurry 203 in the tank 201, or during initialmixing of the slurry 203 ingredients, or through a combination of thesetechniques.

[0031] In connection with the corner trim board 100 of this invention,an alternate press operation 400 is shown in FIG. 7. In this operation400, a plurality of 4 inch wide by foot long strips of fiber cementboard, such as the first set of fiber cement board strips 406, 407 andsecond set of fiber cement board strips 408 and 409, are transferredinto a stack and layered between the first and second V-shapedfiberglass molds 412, 413 and first and second V-shaped rigid supportmembers 410, 411. The stack is introduced between a pair of V-shapedsteel platens, 414, 415 and the press is activated to combine the boardstrips together into a bonded corner having a imprinted aestheticsurface thereon. This process may be used to make one, or a plurality ofcorner trim boards 100 of this invention. Following the pressingoperation 400, the nailing flanges 18 and 19 can be formed in a break orthe like.

[0032] Following the pressing and de-watering step 222, the stack 304 offormed boards, fiberglass molds 303 and support members 302 arepre-cured at step 224 in an oven for 12 hours at 140° F. and 100%humidity. The pre-curing step 224 provides the now formed cementitiousboards with sufficient mechanical properties to be transferred. At thisstage the cementitious product is about 15-20% cured. It is thensubjected to a final autoclaving step 226 for 12 hours at 325° F. in asaturated steam environment. The autoclaving step 226 should besufficient to substantially cure the cementitious product to about a99.9% cure. Following the autoclaving step 226, the board is separatedfrom the molds at step 228, and then sent to inventory for a week or twoprior to the dry cutting step 230. Dry cutting is typically accomplishedwith a gang saw including about 10 blades which are capable of cuttingmultiple products or making multiple cuts in a single product. In thisfashion, scallops, staggered edges and individual shapes can be cut intothe cementitious material to provide an aesthetic appearance. Wetcutting with water jet is also suitable for this purpose.

[0033] Following the dry cutting step 230, the cementitious product issealed and primed at step 232. A good PVA or acrylic latex primer coatfollowed by a tinted finish coat helps to provide the corner trim board100 with years of weatherability.

[0034] While the steps enumerated in the flow diagram of FIG. 3 havebeen successful in producing the cementitious products and corner trimboards 100 of this invention, it will be realized that these steps canbe modified or changed from their selected sequence while stillproviding a quality product. For example, cutting could occur in the wetor green state, as opposed to a final cured state of the product toprovide trim details such as circles, cuts between shapes and productedges. The pressing step 222 could be eliminated in favor of mold orbelt curing or the like. The autoclave and separate mold steps 226 and228 could be reversed for example, and as an alternative to the Hatchekprocess, molding processes could be employed, and an extruder could beused instead of an accumulation roll.

[0035] Materials Discussion

[0036] The following preferred materials have been selected formanufacturing the cementitious products of this invention.

[0037] In the preferred embodiment, the rigid support member 11 includesa rigid polymer resin, such as, rigid polyvinyl chloride (“pvc”),fiberglass-reinforced epoxy or polyester, or a metal plate, sheet orlath. Suitable metallic materials include anodized or polymer-coatedaluminum or copper, brass, bronze, stainless steel, or galvanized steel,in plate, sheet or lath form. If aluminum is selected, it should becoated wherever it comes in contact with the cementitious material ofthis invention, since it is prone to attack by alkali compositions.Similarly, carbon steel selections should be coated or galvanized inorder to prevent rusting, especially in the first and second nailingflanges 18 and 19. The metal plate or lath can be roll formed andpunched in order to provide through-holes 33 and fastener receivingholes 32. If a lath, scrim, or mesh construction is used, separate holesmay not be necessary since the open construction of a lath, scrim, ormesh is ideal for mechanically locking with the cementitious layer 22and is easily penetrated by fasteners such as nails and screws. Withlath or scrim constructions, embedding within the cementitious layer 22is an option, in which case, the rigid support member may containcorrugations, grooves perforations or ridges to assist in mechanicallylocking with the cementitious layer 22.

[0038] Aggregates 25, fibers 24, dispersants, and a rheology-modifyingagents can be selectively added to modify the properties of thecementitious layer 22. The cementitious layer 22 most preferablyincludes a known fiber cement composition including wood fiber, silicasand and portland cement, with or without an acrylic modifier. A varietyof additives can be included within the cementitious layer 22, such asorganic binders, dispersants, one or more aggregate materials 25, fibers24, air entraining agents, blowing agents, or reactive metals. Theidentity and quantity of any additive will depend on the desiredproperties or performance criteria of both the cementitious layer 22 aswell as the sheathing or trim product made therefrom.

[0039] Organic binders are simply polymers that when added to waterunder certain conditions form long chains that intertwine and capturethe components of the mixture. As water is removed from the mixture,these long chains solidify and bind the structural matrix. Because ofthe nature of these organic binders, however, they also function tomodify the rheology of a composition. Whether the organic material is abinder, or primarily affects the rheology is a matter of degree and isdependent on the concentration. In smaller amounts the organic materialprimarily affects the rheology. As the amount of organic material isincreased, its ability to bind the particles together increases,although it also continues to affect the rheology.

[0040] Organic binders can also be added to increase the cohesivestrength, “plastic-like” behavior, and the ability of the mixture toretain its shape when molded or extruded. They act as thickeners andincrease the yield stress of the inorganically filled mixture, which isthe amount of force necessary to deform the mixture. This creates high“green strength” in the molded or extruded product. Suitable organicbinders include a variety of cellulose-, starch-, and protein-basedmaterials (which are generally highly polar), all of which assist inbridging the individual particles together.

[0041] Dispersants, on the other hand, act to decrease the viscosity andthe yield stress of the mixture by dispersing the individual aggregates25, fibers 24, and binding particles. This allows for the use of lesswater while maintaining adequate levels of workability. Suitabledispersants include any material which can be absorbed onto the surfaceof the binder particles or aggregates and which act to disperse theparticles, usually by creating a charged area on the particle surface orin the near colloid double layer. The binders and dispersants can beintroduced in the dry mixing step 210, slurry forming step 212 and/orsprayed between layers 204 by a spray head 208 onto the accumulator roll202, for example.

[0042] It may be preferable to include one or more aggregate materialswithin the cementitious layer 22 in order to add bulk and decrease thecost of the mixture. Aggregates often impart significant strengthproperties and improve workability. An example of one such aggregate isordinary silica sand or clay, which are completely environmentally safe,extremely inexpensive, and essentially inexhaustible.

[0043] In other cases, lightweight aggregates can be added to yield alighter, and often more insulating, final product. Examples oflightweight aggregates are perlite, vermiculite, hollow glass spheres,aerogel, xerogel, pumice, and other lightweight rocklike materials.These aggregates are likewise environmentally neutral and relativelyinexpensive.

[0044] Fibers may be added to the cementitious layer 22 in order toincrease the interlaminar bond strength, compressive, tensile, flexural,and cohesive strengths of the wet material as well as the hardenedarticles made therefrom. Fiber should preferably have high tear andburst strengths (i.e., high tensile strength), examples of which includewaste paper pulp, abaca, southern pine, hardwood, flax, bagasse (sugarcane fiber), cotton, and hemp. Fibers with a high aspect ratio of about10 or greater work best in imparting strength and toughness to themoldable material.

[0045] From the foregoing, it can be realized that this inventionprovides reinforced cementitious sheathing products which are lighter inweight and more resistant to cracking than currently availablecommercial fiber cement products. The preferred corner trim board ofthis invention can use less than half of the cementitious material of aconventional trim board, but since it is reinforced with a rigid supportmember, it will be easier to work with and provide potentially greaterdurability. The cementitious layers of this invention can be joined tothe rigid support member with mechanical and/or adhesive bonds, and theindividual layers of the cementitious products of this invention can befurther reinforced with Theological modifying agents to increase ILBstrength by allowing fibers to displace and flow better across thelaminated boundaries of the cementitious materials, or by adding mortaror cement bonding agents for adhesively bonding these layers together,or both. Although various embodiments have been illustrated, this is forthe purpose of describing, and not limiting the invention. Variousmodifications, which will become apparent to one skilled in the art, arewithin the scope of this invention described in the attached claims.

What is claimed:
 1. A fiber cement trim board comprising: (a) anelongated rigid support member having a wall-facing side and anexterior-facing side, a pair of lateral side portions and a pair oflongitudinal ends, said exterior-facing side including at least a firstand a second exterior wall portion thereon; (b) a first nailing flangedisposed along at least a first of said pair of lateral side portions;(c) a fiber cement layer disposed at least on a portion of saidexterior-facing side of said rigid support member; said fiber cementlayer exhibiting an aesthetic appearance.
 2. The fiber cement trim boardof claim 1 further comprising a second nailing flange disposed along asecond of said pair of lateral side portions.
 3. The fiber cement trimboard of claim 2 wherein said exterior-facing side comprises third andfourth exterior wall portions connected to said first and secondexterior wall portions, respectively.
 4. The fiber cement trim board ofclaim 3 wherein the end portions of said third and fourth exterior wallportions are connected to said first and second nailing flanges,respectively, whereby said first and fourth exterior wall portions aresubstantially parallel and said second and third exterior wall portionsare substantially parallel, and said first nailing flange issubstantially parallel to said first exterior wall portion and saidsecond nailing flange is substantially parallel to said second exteriorwall portion.
 5. The fiber cement trim board of claim 1 wherein saidrigid support member comprises polymer resin, copper, brass, bronze,stainless steel aluminum or carbon steel.
 6. The fiber cement trim boardof claim 1 wherein said rigid support member comprises galvanized steellath, mesh or scrim, or galvanized and perforated steel sheet or plate.7. The fiber cement trim board of claim 1 wherein said rigid supportmember comprises a cement bond promoter disposed on said exterior-facingside thereof.
 8. The fiber cement trim board of claim 6 wherein saidcement bond promoter comprises an acrylic-based material.
 9. The fibercement trim board of claim 1 wherein said fiber cement layer is disposedover said rigid support layer in a thickness of about 0.25-0.75 in. 10.The fiber cement trim board of claim 1 wherein said fiber cement layercomprises a first and second exterior-facing wall portions forming acorner.
 11. A method of manufacturing fiber cement trim board,comprising: (a) forming a rigid support member having a wall-facing sideand an exterior-facing side, a pair of lateral sides and a pair oflongitudinal ends, said forming step producing at least first and secondexterior wall portions; (b) disposing a fiber cement layer on a portionof said exterior-facing side of said rigid support member, said fibercement layer exhibiting an aesthetic appearance.
 12. The method of claim1 wherein said forming step (a) comprises roll forming galvanized steelstock.
 13. The method of claim 1 wherein said forming step (a) comprisesbending a metal lath.
 14. The method of claim 12 wherein said formingstep (a) further comprises punching a plurality of nail slots in saidroll formed galvanized steel stock.
 15. The method of claim 10 whereinsaid disposing step (b) comprises layering said fiber cement layer ontosaid exterior-facing side of said rigid support member, followed bypermitting said fiber cement layer to set.
 16. A fiber cement trim boardcomprising: (a) an elongated rigid support member comprising galvanizedsteel, said rigid support member having a wall-facing side and anexterior-facing side, said exterior facing side of said rigid supportmember including first and second pairs of generally parallel wallsforming a rectangular enclosure having a longitudinal slotted opening;(b) a pair of nail flanges extending from the walls of said rectangularenclosure proximate to said longitudinal slotted opening, said nailflanges having a plurality of fastener receiving holes disposedtherethrough, said rigid support member also including a plurality ofthrough-holes distributed among said pair of parallel walls; and (c) afiber cement layer disposed on a portion of said exterior-facing side ofsaid rigid support member, said fiber cement layer being also disposedthrough at least a portion of said plurality of through-holes in saidpair of parallel walls for forming a mechanical locking engagement withsaid rigid support member.
 17. The fiber cement trim board of claim 15wherein said rigid support member comprises galvanized steel lath orgalvanized and perforated steel sheet or plate.
 18. The fiber cementtrim board of claim 15 further comprising a resinous cement bondpromoter disposed on said exterior-facing side to assist in bonding saidfiber cement layer to said rigid support member.
 19. The fiber cementtrim board of claim 15 wherein said fiber cement layer comprisescellulosic fiber, silica sand and portland cement.
 20. A cementitiousexterior sheathing product comprising: (a) a rigid metal support memberhaving a wall-facing side and an exterior-facing side, a pair of lateralsides and a pair of longitudinal ends, said rigid support memberincluding a nail flange disposed along one of its lateral sides; (b) acementitious layer disposed at least on a portion of saidexterior-facing side of said rigid support member, said cementitiouslayer exhibiting an aesthetic appearance.
 21. The sheathing product ofclaim 19 wherein said exterior-facing side of said metal support membercomprises a cement bond promoter.
 22. The sheathing product of claim 20wherein said cementitious layer comprises fiber cement.
 23. Thesheathing product of claim 20 wherein said cementitious layer is joinedto said metal support member by at least a mechanical lockingengagement.
 24. The sheathing product of claim 20 wherein said metalsupport member is galvanized steel.
 25. The sheathing product of claim20 wherein said metal support member is embedded within saidcementitious layer.